The present invention is concerned with a process for separating isobutylene from mixtures of C.sub.4 hydrocarbons by sulfuric acid extraction.
The state of the art of isobutylene refining and purification may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", Vol. 3 (1964), pages 830-865, under the section "Butylenes", particularly pages 836-839, and FIG. 1 on page 837, and U.S. Pat. No. 2,961,472, the disclosures of which are incorporated herein.
It is known that tertiary olefins will react more readily with sulfuric acid than primary or secondary olefins of the same C number. It is further known that this differential reactive behavior of tertiary olefins with respect to sulfuric acid is used on an industrial scale, mostly for selectively separating isobutylene from C.sub.4 hydrocarbons containing isobutylene and butenes.
Mixtures of C.sub.4 hydrocarbons containing olefin- and paraffin C.sub.4 hydrocarbons are obtained industrially from the thermal and catalytic cracking of petroleum products and from the ethylene production by pyrolysis of light benzene or petroleum fractions of higher boiling points.
According to Kirk-Othmer ibid, Vol. 3 (1964), p. 834, typical compositions of recovered C.sub.4 fractions have:
______________________________________ Component of C.sub.4 Fraction Mole % ______________________________________ isobutane 26.0-38.5 n-butane 6.5-9.5 isobutylene 13-19 1-butene 9-12 2-butene 27-37 ______________________________________
Several processes for selective separation of isobutylene from mixtures of C.sub.4 hydrocarbons containing isobutylene and butenes, and based on the reaction of isobutylene with sulfuric acid, are known.
The oldest sulfuric acid process is the so-called Cold Acid Polymerization Process. In the latter, isobutylene is separated by extracting with 60 - 70 percent sulfuric acid by weight. The extracted isobutylene is removed from the extraction solution as a mixture of dimers and trimers. The 60 - 70 percent by weight recovered sulfuric acid is again used for extraction of isobutylene. Isobutylene may be prepared from the mixture of dimers and trimers by depolymerization (Oil Gas 36,26, 133-42, 1937).
Later the so-called Esso Process was developed. In the latter, isobutylene is separated by means of 60 - 70 percent by weight sulfuric acid in a two-stage extraction process at two different temperatures. Then the mixture of C.sub.4 hydrocarbons in the liquid phase is made to react in countercurrent flow with the acid. Prior to further processing, the extract so obtained is diluted to 45 percent by weight. When the diluted extract is recovered, isobutylene is obtained in a purity of 96 - 99 percent by weight and sulfuric acid as 45 percent by weight. The sulfuric acid so obtained must be concentrated to 60 -70 percent by weight prior to renewed use in extracting (Petroleum Refiner, 33,5, 156-9, 1954; also German Pat. No. 829,295). Processes improving the recovery of the extracted isobutylene similar to the Esso process itself yield only unsatisfactory amounts of and purities of isobutylene as disclosed in U.S. Pat. No. 2,961,472.
According to U.S. Pat. No. 2,961,472, theoretically the extraction process using concentrated aqueous solutions of sulfuric acid merely consists in the commercial exploitation of the following well known chemical reaction:
______________________________________ H.sub.2 SO.sub.4 i - C.sub.4 H.sub.8 + H.sub.2 O .revreaction. t - C.sub.4 H.sub.9 OH ______________________________________
The tertiary butyl alcohol is soluble in sulfuric acid, and so the sulfuric acid phase, containing the dissolved tertiary butyl alcohol, is separated from the hydrocarbon phase. Since the original reaction is reversible and isobutylene is the desired end product, the isobutylene may be recovered by the application of heat to the sulfuric acid solution of the tertiary butyl alcohol.
The process of U.S. Pat. No. 2,961,472 is an improvement over the process of selectively extracting isobutylene from a mixture of C.sub.4 hydrocarbons comprising olefins and paraffins in addition to the isobutylene by reacting the C.sub.4 hydrocarbon mixture with an aqueous solution of sulfuric acid to convert the isobutylene to tertiary butyl alcohol soluble in the aqueous sulfuric acid. The sulfuric acid solution is separated from the C.sub.4 hydrocarbon mixture and the tertiary butyl alcohol in the sulfuric acid solution is regenerated to isobutylene. The extraction sulfuric acid from the regeneration is reconcentrated by vacuum distillation and has a maximum of 2.0 percent by weight of non-aqueous impurities therein. The sulfuric acid solution containing tertiary butyl alcohol and physically entrained and dissolved hydrocarbon impurities is passed to a venting zone maintained at a temperature of 135.degree.- 150.degree.F prior to the regeneration of the isobutylene.
With regard to a further process, the so-called CFR process, 50 percent by weight sulfuric acid is used. Isobutylene is separated in a multi-stage extraction process in a manner similar to and under similar conditions as in the Esso process. There is no dilution of the extract obtained prior to recovery. The latter yields isobutylene of a purity exceeding 99 percent by weight and 50 percent by weight sulfuric acid, this 50 percent sulfuric acid being used again for extraction (Erdoel und Kohle, 16, 100-4, 1963).
Lastly, BASF has developed a process wherein 45 percent by weight sulfuric acid is used for obtaining isobutylene; a mixture of C.sub.4 hydrocarbons in the gaseous state is made to react with sulfuric acid in bubble columns. Tertiary butanol is isolated at temperatures up to 50.degree.C and at lowered pressures from the extraction solution obtained. The recovered acid is then used again for extraction. The isolated tertiary butanol is converted into isobutylene of a purity exceeding 99.9 percent by weight by means of a dehydration catalyst (Erdoel und Kohle 22, 605-8, 1969).
The previously known processes yield isobutylene and refined C.sub.4 hydrocarbons of varying qualities. The purer the isobutylene obtained from these processes, the higher the isobutylene content in the refined C.sub.4 hydrocarbons. The latter factor is a drawback if highly pure isobutylene and refined C.sub.4 hydrocarbons practically free from isobutylene are simultaneously required. Refined C.sub.4 hydrocarbons containing more than 0.5 percent by weight of isobutylene are virtually unusable for certain applications such as the preparation of secondary butanol, or of methylethyl ketone and 1-butene with a purity exceeding 99 percent by weight.